Can we just switch off a malicious artificial intelligence?
Malware viruses are a very simple form of AI.  It is not difficult to conceive of a form of malware that A) can't be detected easily, B) is redundantly distributed across thousands of computers that occasionally connect to the internet, C) is capable of detecting some kinds of threats to itself and mutating to avoid the threats.

So, simply "turning off" a malicious AI will not always be possible.